(1) Field of the Invention
The present invention relates to a code division multiple access communication system used in land mobile communications such as mobile telephones and portable telephones and in particular to a communication system using a code division multiple access communication system for utilizing a limited frequency band efficiently.
(2) Description of the Prior Art
A demand for land mobile communications such as mobile telephones and portable telephones has remarkably increased, and therefore, efficient frequency utilization techniques for keeping more subscribers in a limited frequency band are important. One of the multiple access systems for efficient frequency utilization is a code division multiple access communication system (CDMA system). A CDMA system is a multiple access system utilizing a spread spectrum communication technique which is resistant to multipath distortion and can be expected for a diversity effect by a RAKE receiver which combines multipath components in a maximal ratio. A land mobile communication system using a CDMA system is disclosed, for example, in U.S. Pat. No. 4,901,307.
U.S. Pat. No. 4,901,307 describes a CDMA communication technique in the case in which a plurality of subscribers communicate through base stations. In a CDMA system, a system in which all base stations transmit the same pilot signal in frequency and spread code is well known. In U.S. Pat. No. 4,901,307, a pilot signal is used as a reference for initial synchronization, carrier phase offset, and carrier frequency offset at a mobile station and a reference time frame is transmitted from a base station. Propagation loss from each base station can be determined by detecting the pilot signal.
In a mobile communication system using a CDMA system, when a level of the signal from the mobile station near the base station on the reverse link from the mobile station is high, there occurs a near-far problem which causes signals from the other mobile equipment to not be received. Accordingly, it is necessary to control the transmission power of the mobile station so that the base station can receive a signal with the same level from every mobile station. Such a control method of a transmission power is described, for example, in U.S. Pat. No. 5,056,109 and 41st Vehicular Technology Conference (May, 1991 pp. 57-62).
In a mobile communication system using a CDMA system, it is possible to use the same frequency band between adjacent cells and utilize frequency resources efficiently. When the same frequency is used between adjacent cells, soft handover between cells is possible and it is possible to improve communication quality near a cell boundary. The soft hand over technique is described in U.S. Pat. No. 5,101,501.
In a typical CDMA system, however, communication quality is sometimes deteriorated by delays in the control of transmission power. In a microcell environment, propagation loss may suddenly vary due to shadowing. Communication quality deterioration occurs especially in all mobile stations communicating with an adjacent base station for a long time. To solve this problem, there is a method for maintaining the performance of the entire system by rapidly reducing the transmission power of the mobile station creating some interference and allowing only communication quality deterioration of the mobile station. But in this case, the mobile station cannot execute a soft hand over and a call drop could occur. As a result, the communication quality of the mobile station significantly deteriorates for a long time. The mobile station suddenly reducing its own transmission power is known as "self-sacrifice".